Example configurations of conventional AC motor drive systems include an AC motor drive system in which an inverter and a charging and discharging circuit are connected in parallel with each other to a DC bus on the output side of a converter that converts AC power from a system power supply into DC power via a smoothing capacitor. The inverter converts DC power into AC power of a voltage value and a frequency different from those of the system power supply to drive the AC motor, and the charging and discharging circuit is used for charging and discharging a power storage device that stores and releases DC power.
For example, Patent Literature 1 discloses a technique of controlling, on the basis of the voltage value in a DC bus that rises due to the regenerative power from an AC motor, the peak of power regenerated for a system power supply via a converter by charging a power storage device with part of the regenerative power, and a technique of, on the basis of the DC bus voltage value that drops due to the power consumption of the AC motor during powering of the AC motor, controlling the peak of the power supplied from the system power supply via the converter by discharging power stored in the power storage device. According to the techniques described in Patent Literature 1, there is a distinct difference in the states where the DC bus voltage value rises during regeneration of the AC motor and where the DC bus voltage value drops during powering of the AC motor when compared with the DC bus voltage value when the AC motor is performing neither a powering operation nor a regenerative operation, i.e., when the AC motor is unloaded. On the basis of this difference, the peaks of powering power passing from the system power supply to the AC motor through the converter and regenerative power passing from the AC motor to the system power supply through the converter are controlled by charging and discharging the power storage device. For lowering the costs and extending the useful lives of components of a smoothing capacitor, a converter, and an inverter connected to a DC bus by relaxing withstand voltage specifications, it is preferable to reduce the rise of the DC bus voltage value during AC motor regeneration or to prevent the rise, and it is more preferable that the DC bus voltage value drop below that at no-load time.